Scientists capture the shadow cast by a single atom

A team of researchers at Griffith University has managed to stretch the capabilities of microscopy to its ultimate limit. Culminating a five-years effort, the scientists have obtained a digital image of the shadow cast by a single atom, in a development that might soon lead to important advances in scientific observations ranging from the very big to the very small.

Holding an atom in place long enough to take its picture has been within our technological grasp for some time. This is done by isolating the atom inside a chamber and holding it still through electrical forces, a method known as a radiofrequency Paul Trap (named after Wolfgang Paul, who shared the Nobel Prize in Physics in 1989 for this work).

The researchers trapped single ytterbium ions using this technique and exposed them to a very specific frequency of laser light. Under this light, the atom's shadow was cast onto a detector and then captured by a digital camera. This was possible because of a super high-resolution microscope, which makes the shadow dark enough to see. No other facility in the world sports a resolution high enough to allow for such an extreme feat.

The process requires extreme precision, as changing the frequency of the light illuminating the atom by just one part in a billion is already enough to make the shadow disappear.

"Atoms only respond to very specific light frequencies, and these frequencies are different for each element. The very fine frequency control that we use is a fairly standard feature of modern atomic physics experiments," Professor Kielpinski, who led the research efforts, told Gizmag. The breakthrough pushes microscopy to its ultimate limit because, as Kielpinksi explained, it is impossible to see anything smaller than an atom using visible light.

But the researchers' ultimate goal wasn't just to take a simple picture. Absorption imaging plays a fundamental role in modern scientific research, from astronomical observations of dust clouds to biomicroscopy. Measuring how much light a single atom can absorb is crucial to understanding exactly how far scientists can stretch the limits of this imaging technique.

Using their results, the researchers can now predict how much light an atom should absorb when forming a shadow, measure whether the microscope is achieving maximum contrast, and adjust their parameters accordingly to achieve the best possible image quality without damaging the samples. This is important because an excessive amount of X-rays or UV light could damage fragile biological samples, such as DNA strands.